Measurement Principle and Precautions of Ground Resistance

　　In order to discharge the electric charge into the ground, it is necessary to bury a metal conductor (grounding body) in the soil, and connect a part of the electrical equipment to the grounding body with a wire (grounding wire), which is called grounding. The grounding body and the grounding wire are collectively called a grounding device.
　　Several methods of grounding
　　1. Protective grounding. When electrical equipment leaks electricity due to insulation or other accidents, its metal casing may be charged. In order to prevent electric shock accidents, the casing of the electrical equipment must be grounded, which is called protective grounding.
　　2. Lightning protection grounding. The lightning arrester grounding of tall buildings is called lightning protection grounding. 
　　3. Working grounding. Electrical equipment needs to ground a certain point in the circuit (usually the neutral point) for normal operation or troubleshooting, which is called working grounding.
　　The way to ground the equipment is usually to bury metal grounding piles, metal mesh and other conductors, and the conductors are then connected to the ground wire bar or casing in the equipment through cables. When multiple devices are connected to the same grounding conductor, it is usually necessary to install a grounding bar. The location of the grounding bar should be as close to the grounding pile as possible, and the ground wires of different devices are connected to the grounding bar separately to reduce mutual influence.
　　Principle of grounding resistance measurement
　　There are many factors that affect grounding resistance, such as the size (length, thickness), shape, quantity, burial depth of grounding piles, surrounding geographical environment (such as flat land, ditch, slope land are different), soil moisture, texture, etc., which may affect grounding resistance. The
　　grounding resistance tester we use is a more traditional measuring instrument. Its basic principle is to use the three-point voltage drop method. Its measurement method is to drive two auxiliary test piles into the ground on one side of the grounding pile of the measured ground wire (temporarily called X). The two test piles are required to be on the same side of the measured ground pile. The three are basically in a straight line. The auxiliary test pile closer to the measured ground pile (called Y) is about 20 meters away from the measured ground pile, and the auxiliary test pile farther from the measured ground pile (called Z) is about 40 meters away from the measured ground pile. When testing, set the gear to 3P gear. Press the test key. At this time, a voltage can be obtained between the measured ground pile X and the auxiliary ground pile Y. The instrument can calculate the ground resistance of the measured ground pile by measuring the current and voltage values.
　　Precautions in measurement
　　1. The magnitude of the grounding resistance value is related to environmental factors such as season, weather, soil dryness and wetness, and varies with the changes of the above factors. Generally speaking, it is suitable to measure the grounding resistance in autumn, when the weather is clear and dry, and the measured value is more accurate and reliable.
　　2. When measuring the protective grounding resistance of electrical equipment, be sure to disconnect the equipment, otherwise it will affect the measured value.
　　3. In the measurement, it is often found that some grounding wires are seriously rusted over the years. The rust must be removed with a file first, and the wires must be in good contact before measurement. Otherwise, the measurement will be distorted due to the large contact resistance.
　　4. Pay attention to whether it is single-point grounding, whether the measured ground wire has been connected to the equipment, and whether there is a reliable grounding loop. Open-circuit grounding piles cannot be measured; unreliable grounding loops will result in inaccurate (high) measurement results.
　　5. Pay attention to the measurement position and select appropriate measurement points. Different measurement points will produce different results, and sometimes the difference is very large. According to the working principle of the clamp-type ground resistance meter, this is not difficult to understand, which requires attention to the selection of measurement points during use. Sometimes, when measuring, there is no place to clamp. If conditions permit, the original ground wire connection can be temporarily disconnected and a section of clampable jumper wire can be temporarily connected for measurement.
　　6. Pay attention to "noise" interference. The larger loop current on the ground wire will interfere with the measurement, resulting in inaccurate measurement results, or even making the test impossible. Many instruments will display "Noise" or similar symbols in this case.
　　◆ Extended reading
　　After measuring the grounding resistance of the grounding points of industrial and mining enterprises, workshops, and high-rise buildings, a test report should be issued. The test report should indicate the test time, weather conditions, test location, grounding method, soil dryness and wetness, measured grounding resistance value, and whether it meets the safety requirements.
　　Generally speaking, the qualified range of grounding resistance values ​​for protective grounding and working grounding of electrical equipment is generally required to be less than 4 ohms; and the grounding resistance value of lightning protection grounding of lightning rods in high-rise buildings is required to be less than 10 ohms. For grounding points that do not meet the requirements after testing, they should be reported in time and supervised for rectification, and the grounding electrodes should be reburied, or the grounding wires should be checked to ensure safety.